Leukocytes including monocytes, macrophages, basophils, and eosinophils play important roles in the pathological mechanisms initiated by T and/or B lymphocytes. Macrophages, in particular, produce powerful oxidants and proteases which contribute to tissue destruction and secrete a range of cytokines which recruit and activate other inflammatory cells.
The investigation of the critical, regulatory processes by which white cells proceed to their appropriate destination and interact with other cells is underway. The current model of leukocyte movement or trafficking from the blood to injured or inflamed tissues comprises the following steps. The first step is the rolling adhesion of the leukocyte along the endothelial cells of the blood vessel wall. This movement is mediated by transient interactions between selectins and their ligands. A second step involves cell activation which promotes a more stable leukocyte-endothelial cell interaction mediated by the integrins and their ligands. This stronger, more stable adhesion precipitates the final steps—leukocyte diapedesis and extravasation into the tissues.
The chemokine family of polypeptide cytokines, also known as intercrines, possesses the cellular specificity required to explain leukocyte trafficking in different inflammatory situations. First, chemokines mediate the expression of particular adhesion molecules on endothelial cells; and second, they generate gradients of chemoattractant factors which activate specific cell types. In addition, the chemokines stimulate the proliferation of specific cell types and regulate the activation of cells which bear specific receptors. Both of these activities demonstrate a high degree of target cell specificity.
The chemokines are small polypeptides, generally about 70–100 amino acids (aa) in length, 8–11 kD in molecular weight and active over a 1–100 ng/ml concentration range. Initially, they were isolated and purified from inflamed tissues and characterized relative to their bioactivity. More recently, chemokines have been discovered through molecular cloning techniques and characterized by structural as well as functional analysis.
The chemokines are related through a four cysteine motif which is based primarily on the spacing of the first two cysteine residues in the mature molecule. Currently the chemokines are assigned to one of two families, the C-X-C chemokines (α) and the C-C chemokines (β). Although exceptions exist, the C-X-C chemokines generally activate neutrophils and fibroblasts while the C-C chemokines act on a more diverse group of target cells which include monocytes/macrophages, basophils, eosinophils, T lymphocytes and others. The known chemokines of both families are synthesized by many diverse cell types as reviewed in Thomson A. (1994) The Cytokine Handbook, 2nd Ed., Academic Press, N.Y. The two groups of chemokines will be described in turn.
The archetypal and most extensively studied C-X-C chemokine is platelet factor 4 (PF4). This 70 aa protein displays the definitive four cysteines and is released along with platelet derived growth factor (PDGF), transforming growth factor β (TGF-β) and β-thromboglobulin (β-TG) from the granules of stimulated platelets. This homotetrameric molecule shares structural similarity with interleukin-8 (IL-8), induces the migration of fibroblasts, neutrophils and monocytes, and binds heparin. PF4 provides the biological model for a link among thrombosis, inflammation, and wound healing.
Other chemokines found in the platelet a granule include β-TG, connective tissue activating protein III (CTAP-III) and neutrophil activating peptide 2 (NAP-2). All three peptides are derived from the differential processing of a precursor molecule, platelet basic protein (PBP). β-TG is an 81 aa, highly basic protein which influences the migration of fibroblasts but has no effect on neutrophils or monocytes. CTAP-III is 85 aa long, and aa 4–85 are identical to β-TG. Since CTAP-III is the primary protein in the a granule and its role as a purified protein has not been elucidated, it may be a secondary precursor, inactive until further processed. NAP-2 appears to attract neutrophils but not monocytes.
Nonplatelet C-X-C chemokines include IL-8, γ interferon inducible protein (IP-10), melanocyte growth stimulatory activity (MGSA or gro) proteins, epithelial derived neutrophil attractant-78 (ENA-78), granulocyte chemotactic protein-2 (GPC-2) and stromal cell-derived factors-1α and 1β (SDF-1α and -1β). IL-8 (also known as NAP-1) is secreted by monocytes/macrophages, neutrophils, fibroblasts, endothelial cells, keratinocytes and T lymphocytes in response to proinflammatory cytokines, IL-1 and 3, IFN-γ and TNF, as well as endotoxins, mitogens, particulates, bacteria and viruses. IL-8 stimulates acute inflammation including the upregulation of both neutrophil adhesion and keratinocyte growth and the downregulation of histamine production by basophils.
IP-10 is a 10 kD protein of undefined function whose mRNA has been found in monocytes, fibroblasts and endothelial cells. Monocytes, keratinocytes and activated T cells secrete IP-10 protein which has been localized to sites of delayed hypersensitivity reactions. The cDNA of MGSA/gro a produces a 15 kD protein which appears in fibroblasts. Its transcription is growth related, and it functions as an autocrine growth factor. The distinct and non-allelic forms, gro β and gro γ, are 90% and 86% identical to gro α, respectively. Recombinant gro a proteins attract and activate neutrophils. ENA-78 was purified from supernatants of lung alveolar cells. Like gro α, it attracts and activates neutrophils in vitro.
GCP-2 is a 6 kD protein isolated from the supematants of osteosarcoma cells. GCP-2 exists in various N-terminally truncated forms, and it attracts and activates neutrophils in vitro and causes granulocyte accumulation in vivo. SDF-1α and -1β are newly isolated cDNAs which encode secreted molecules and type I membrane proteins.
Current techniques for diagnosis of abnormalities in inflamed or diseased tissues mainly rely on observation of clinical symptoms or serological analyses of body tissues or fluids for hormones, polypeptides or various metabolites. Patients often manifest no clinical symptoms at early stages of disease or tumor development. Furthermore, serological analyses do not always differentiate between invasive diseases and genetic syndromes which have overlapping or very similar ranges. Thus, development of new diagnostic techniques comprising small molecules such as the expressed chemokines are important to provide for early and accurate diagnoses, to give a better understanding of molecular pathogenesis, and to use in the development of effective therapies.
The chemokine molecules were reviewed in Schall T J (1994) Chemotactic Cytokines: Targets for Therapeutic Development, International Business Communications, Southborough, Mass., pp 180–270; and in Paul W E (1993) Fundamental Immunology, 3rd Ed., Raven Press, NYC, pp 822–826.